Apparatuses for immersion lithography typically include a substrate chuck, a reservoir for an immersion liquid and a pattern-transferring system. The pattern-transferring system typically includes a photomask and optical lenses above and beneath the photomask. During an exposure process in immersion lithography, radiation may pass through the pattern-transferring system and the immersion liquid to the photoresist layer on a substrate provided on the substrate chuck. During this exposure step of the immersion lithography process, the passing of the radiation through the immersion liquid may increase the depth-of-focus relative to photolithography performed in air, which may allow for a more effective transfer of the photomask's pattern to the photoresist layer.
However, other challenges may result in immersion lithography. For example, residues from the photoresist layer may leach into the immersion liquid. Such photoresist residues present in the immersion liquid may diffuse-reflect the exposing radiation and may subsequently become deposited on the substrate chuck and its peripheral elements.
A “dummy” layer may be used to prevent or decrease the deposition of photoresist residues on immersion lithography apparatuses. For example, before the exposure step in an immersion lithography process, a dummy layer may be deposited on the substrate chuck, and then a pre-immersion lithography process may be performed thereon while the dummy layer is in contact with the immersion liquid.
Typically, the dummy layer includes a dummy substrate, which is generally formed of single-crystalline silicon that may a native oxide layer formed thereon, often irregularly, due to the environment of semiconductor manufacturing lines. The immersion liquid may be water. In such cases, the native oxide layer may be relatively hydrophilic, while the single-crystalline silicon of the dummy substrate may be relatively hydrophobic. Thus, water droplets may irregularly form on the dummy layer during and/or after the pre-immersion lithography process. Since water droplets remaining on the dummy layer may have a greater affinity for the native oxide layer relative to the dummy layer, evaporation from the dummy layer per unit time may not be uniform across the surface, and thus the surface temperature of the substrate chuck covered by the dummy layer may not be uniform.
Additionally, water droplets may form on the immersion lithography apparatus during the pre-immersion lithography process, and then photoresist residues in the water droplet may adhere to the substrate chuck and its peripheral elements. The water droplets and the photoresist residues therein remaining on the substrate chuck and its peripheral elements may also reduce the reliability of the immersion lithography apparatus.
An immersion lithography apparatus is described in U.S. Patent Publication No. 2006/0033898 A1 to Cadee et al. According thereto, the apparatus limits the immersion liquid to a predetermined region of the substrate using a water supply system, barrier layer and an element. However, such an apparatus does not allow for the removal of photoresist residues before or after an immersion lithography process, nor does it describe a method of maintaining constant surface temperature across the substrate chuck.